Recently, a magnetic memory (MRAM: Magnetic Random Access Memory) is attracting attention as a nonvolatile memory that can be programmed an infinite number of times. The MRAM includes an MTJ (Magnetic Tunnel Junction) element using the tunnel magnetoresistive effect as a memory cell.
The MTJ element has a structure in which a plurality of ferromagnetic layers are stacked with a tunnel barrier layer being interposed between them. In the MTJ element, an electric current flows by tunneling through the tunnel barrier layer. In a data write operation, the magnetization directions in two ferromagnetic layers are relatively reversed into a parallel state or anti-parallel state by supplying a write current. In a read operation, the change in resistance value in the parallel state or anti-parallel state is detected by supplying a read current. More specifically, binary data is detected by a current value by regarding that the anti-parallel state in which the resistance value is maximum is a “1” state, and the parallel state in which the resistance value is minimum is a “0” state. In the MRAM, the time required for magnetization reversal is very short. Therefore, the access time of data read/write operation can be made shorter than that of a DRAM (Dynamic Random Access Memory).
The MRAM is expected as a nonvolatile large-capacity memory capable of high-speed read, and presumably used in a relatively severe environment such as a mobile memory of a mobile apparatus or a main memory of a personal computer.
Unfortunately, the MRAM has the problem that the read/write characteristics largely change (deteriorate) with respect to the temperature because data read/write is performed by controlling the magnetization directions in the ferromagnetic layers. Especially when the temperature decreases, the magnetization reversing energy of the ferromagnetic layer increases due to the magnetostriction effect. Consequently, the write characteristic at a low temperature is inferior to that at room temperature. This makes it necessary to study an MRAM having high reliability even when the temperature changes.